Model of inner-sphere adsorption of oxyanions in goethite - Why is phosphate adsorption more significant than that of sulfate?

ABSTRACT

Phosphorus availability in soils is low due to its strong retention by inner-sphere complexation on minerals in the clay fraction with pH-dependent charges, such as goethite. On the other hand, sulfur has greater availability because it is retained mainly by electrostatic attraction. We evaluated the intensities of the inner-sphere complexation of orthophosphate and sulfate (H₂PO^4-/HPO₄^2- and SO₄^2- - generically treated as PO₄ and SO₄) under different experimental conditions (pH, goethite purity, and contact times) on synthetic goethite samples to establish the mechanisms and models involved in those bonds. Inner-sphere PO₄ and SO₄ were extracted using both HNO₃ 1 mol L^-1 and USEPA 3051A methods. Inner-sphere complexation of PO₄ and SO₄ was highest at pH 5 in relation to pH 9. Attenuated total reflectance/Fourier transform infrared spectroscopy (ATR-FTIR) spectra showed inner-sphere complexation bands of PO₄ on goethite in the protonated binuclear bidentate (pH 5) and deprotonated binuclear bidentate (pH 9) forms. Inner-sphere complexation of PO₄ was much more expressive than that of SO₄. Phosphorus and sulfur oxyanions displace the diprotonated ferrol ligand (-OH₂^+0.5 in -FeOH₂^+0.5), while the -OH^-0.5 in the -Fe-OH^-0.5 group are only displaced by PO₄. The -O^-1.5 ligand in Fe-O^-1.5 group is not displaced by PO₄ or SO₄. The high surface negative charge density of PO₄ defined its higher activation energy for exchanging -OH₂^+0.5 and -OH^-0.5 on the goethite surface in relation to SO₄. The proposed model can be used to reduce inner sphere phosphate adsorption in soils and improve P fertilization efficiency for farming.

iron oxyhydroxides; ligand exchange; activation energy; ferrol groups; zero-point charge